Method for making a fuel tank in plastic material

ABSTRACT

Process for manufacturing a plastic fuel tank from two shells formed by molding, which are joined together, at least one shell being produced by compression-molding a portion of a plastic sheet between a mold and a punch and by the remaining portion of the sheet being blow-molded in the region not compression-molded.

The present invention relates to a process for manufacturing a plasticfuel tank.

Plastic fuel tanks on board vehicles of various kinds must generallymeet sealing and permeability standards in relation to the type of usagefor which they are designed and the environmental requirements that theymust respect. At the present time, both in Europe and in the world,there is a substantial tightening of the requirements relating to thelimitation of pollutant emissions into the atmosphere and theenvironment in general. As a consequence, the design of fuel tanks ismoving rapidly toward techniques capable of providing a better guaranteeof sealing and of safety under various operating conditions. Moreover,manufacturers are also striving to reduce as far as possible the lossescaused by various pipes and accessories connected to the tanks. Onemeans sometimes used has been to incorporate certain accessories andpipes inside the tanks, thus eliminating their interface with theexternal environment.

Patent application GB-1 136 613 discloses a process for molding articlesfrom a sheet of thermoplastic resin, comprising the extrusion of thesheet followed by its molding in a two-part mold, the peripheraloutlines of which can press the peripheral parts of the sheet, producinga join capable of maintaining a pressure difference on either side ofthe sheet so as to form the central part.

This known process is, however, applied to the manufacture of openarticles having around their periphery a flat edge, which is optionallyrecessed by a groove. The articles, of quite small size, are suitable ascontainers. They are manufactured with blowing pressures of at most 10kg/cm2. In addition, no device or accessory is introduced before moldingor found in the articles obtained.

Patent U.S. Pat. No. 5,129,544 is known, which discloses a plastic fueltank comprising two hollow shells made of a multilayer material, saidshells being joined together in an impervious manner over their entireperimeter by fastening together their flat peripheral rim. In this tank,the shells may be formed by stamping or by vacuum-forming.

German patent application DE-OS-1 801 966 discloses the manufacture ofhollow bodies made of thermoplastic from two shells which are formed bymolding and joined together by mirror welding. The molding of each ofthe shells is performed by compression-molding a portion of athermoplastic sheet and by blow-molding the remaining portion of thesheet in the region not compression molded.

The aim of the invention is to provide a process for producing a plasticfuel tank with excellent control of the reproducibility and accuracy ofthe dimensions, which may have a large volume of up to about 100 to 150liters and which is well suited to the incorporation of accessoriesbefore the molding phase.

For this purpose, the subject of the invention is a process formanufacturing a plastic fuel tank from two shells formed by molding,which are joined together, at least one shell being produced bycompression-molding a portion of a plastic sheet between a mold and apunch and by the remaining portion of the sheet being blow-molded in theregion not compression-molded.

The term “fuel tank” is understood to denote a sealed tank capable ofstoring fuel under diverse and varied operating and environmentalconditions. An example of this tank is that with which automobiles areequipped.

The fuel tank produced in the process according to the invention is madeof plastic, that is to say of material comprising at least one polymerin the form of a synthetic resin.

All types of plastic may be suitable. Plastics which are very suitablebelong to the thermoplastics category.

The term “thermoplastic” denotes any thermoplastic polymer, includingthermoplastic elastomers, and blends thereof. The term “polymer” denotesboth homopolymers and copolymers (especially binary or ternarycopolymers). Examples of such copolymers are, in a non-limiting manner:random copolymers, alternating copolymers, block copolymers and graftcopolymers.

Any type of thermoplastic polymer or copolymer whose melting point isbelow the decomposition temperature is suitable. Syntheticthermoplastics which have a melting range spread over at least 10degrees Celsius are particularly suitable. Examples of such materialsinclude those which exhibit polydispersity in their molecular weight.

In particular, polyolefins, thermoplastic polyesters, polyketones,polyamides and copolymers thereof may be used. A blend of polymers orcopolymers may also be used, as may a blend of polymeric materials withinorganic, organic and/or natural fillers such as, for example, butnon-limitingly, carbon, salts and other inorganic derivatives, naturalor polymeric fibers. It is also possible to use multilayer structuresconsisting of stacked layers bonded together, comprising at least one ofthe polymers or copolymers described above.

A polymer often employed is polyethylene. Excellent results have beenobtained with high-density polyethylene (HDPE).

According to the invention, the tank is manufactured from two shells.The term “shell” denotes a non-closed partial envelope in the form of anopen hollow body, with a surface shape having at least one concaveportion.

The two shells are joined together in order to obtain the tank. Any typeof impervious joint is suitable. However, it is preferred to join theshells together by welding.

Each shell used has been formed by molding a plastic sheet. A sheetintended for molding may consist of a single plastic composition. It mayalso be in the form of a structure resulting from stacking severallayers of different plastic compositions. A layer often used toadvantage in such a structure is a layer comprising at least onematerial acting as a barrier to liquids and gases, in particular tohydrocarbons.

According to the invention, the molding of at least one shell comprisesthe compression-molding of a portion of the sheet and the blow-moldingof the portion not compression-molded. These two operations areperformed in the same mold.

The compression-molding is carried out by bringing together andclamping, on each side of the sheet, at least one region of a moldportion and of a punch which is fastened to the other mold portion.

The blow-molding is carried out by introducing, on one side of thesheet, into the region not compression-molded, a pressurized fluid. Thisfluid may be a gas, a liquid or a dispersion of at least one liquid in agas. When the fluid comprises a liquid, it advantageously ensures betterheat transfer between the sheet and the fluid. Preferably, thepressurized fluid is introduced on that side of the sheet where thepunch is located. As pressurized fluid, compressed air has given goodresults. It is also possible to use a pressurized purging fluidcontaining a reactive gas. Such a reactive gas may be fluorine. It isalso possible to use an inert gas, such as nitrogen. A mixture ofvarious gases may also be used, in particular a mixture containing atleast two of the abovementioned gases. Among liquids, it may beadvantageous to use water. A fluid that has led particularly toexcellent results is a dispersion (a spray) of water in compressed air.

In the process according to the invention, the molding is well suited tothe treatment of a sheet placed vertically. One example is that of asheet produced by extrusion, in an extruder placed vertically, theextrusion head which includes the die being located at the lowest pointand the sheet produced flowing rectilinearly owing to the effect of itsown weight.

According to the invention, the sheet is obtained in the samemanufacturing line as the shell which will be produced from this sheet,by cutting and opening an extruded parison of closed cross section. Oneof the shells may have been independently manufactured beforehand in amanufacturing unit other than for the tank. For example, the shell mayhave been manufactured in a molding installation different from thatused to manufacture the tank.

However, at least one of the shells used is preferably molded in thesame manufacturing unit as the tank. This shell may, for example, bemolded in an operating phase immediately preceding the production of thetank. It may also be produced in an operating phase which is, at leastpartly, simultaneous with producing the tank. Both shells may also bemolded in the same manufacturing unit as the tank.

In accordance with the process according to the invention, the sheet isadvantageously obtained by cutting and opening a parison of closed crosssection leaving the die mounted on the extrusion head. The cuttingoperation consists in piercing and cutting, through its entirethickness, the wall of the parison along a curve of predetermined shapeand length. Preferably, the cutting curve is rectilinear. Alsopreferably, the cutting is carried out continuously over the entirelength of the parison. Most preferably, the cut is produced as astraight line over the entire length of the parison.

The cut parison may be opened by any suitable means. In particular, thecut parison may be opened by means of guiding devices. Examples of suchdevices are, in a non-limiting manner, wheels and rollers.

According to one particularly preferred method of implementing theprocess according to the invention, that portion of the sheet whichundergoes compression-molding comprises the regions for joining theshells together.

The “joining regions” are understood to mean those portions of the shellwhich are intended to co-operate with that portion of the other shellduring joining so as to produce a sealed tank.

When the two shells are joined together by welding, the joining regionscomprise the weld edges.

One particular method of implementing the process according to theinvention consists in compression-molding and blow-molding the twoshells at the same time, by using stacked double molds, also calledsandwich molds or stack molds.

According to another particularly advantageous method of implementingthe process according to the invention, before joining, at least oneaccessory of the fuel tank is inserted into the shell and fixed onto it.

The term “accessory” is understood to denote any object or devicegenerally associated with the fuel tank in its usual method of use or ofoperation and which co-operates with the fuel tank to provide certainuseful functions. Non-limiting examples of such accessories are liquidpumps, delivery tubes, reservoirs or baffles internal to the fuel tank,venting devices.

Any method of fixing the accessory to the shell which is compatible withthe use of the fuel tank is suitable. One fixing method which has givengood results is welding.

Alternatively, the accessory may, while the shell is being molded, bemolded at the same time as the latter, with the intervention of at leastone particular device.

One useful technique for effecting this simultaneous molding of theaccessory and of the shell is for the sheet serving for manufacturingthe shell to undergo flow, at the very least in a localized region ofthis sheet.

Another method of fixing the accessory in the process according to theinvention comprises the use of an additional device for overmolding theshell, which allows at least one accessory of the fuel tank to be moldedand fixed directly by the addition of material on one side of the wallof the shell. The preferred material is a plastic or a plastic-basedcomposite. A plastic identical to that of the sheet has given goodresults.

The overmolding may, in this way, be carried out on the inside (concaveside) of the shell. It may also be carried out on the outside (convexside). Preferably, this technique is used to mold and fix an accessoryon the inside (concave side) of the shell, because of the incorporation,after the shells have been molded, of the accessory thus produced insidethe tank.

According to this latter particular technique, the addition of materialmay be performed by injection-molding a plastic composition.

It is also possible to combine the fixing of at least one accessory byflow of the sheet, as described above, with that of at least oneadditional accessory by overmolding on the inside.

Another method for fixing an accessory to the inside of the shellcomprises the use of at least one device placed on the punch side or onthe die side of the mold, which is capable of fixing at least one fueltank accessory by welding the latter to the sheet. An alternative fixingtechnique consists in at least partially covering the accessory with aportion of the sheet. Preferably, at least two devices are used.

The die side is that which corresponds in the mold to the outside of theshell and of the tank. Preferably, two devices placed on the punch sideare used, so as to fix an accessory to the shell and to the tank on theinside.

This latter method of fixing an accessory may also be combined with oneor more other methods described above.

In one particular beneficial method of implementing the processaccording to the invention, a pre-assembled structure is inserted duringmolding, allowing the tank to be stiffened by simultaneously pressing oneach of the two shells and/or fixing at least one accessory of the tank.

The preassembled structure is made of a material composition or materialcompatible with the tank. Preferably, it comprises a plastic of the samenature as that of the internal wall of the tank. The preassembledstructure may combine several identical or different accessories via anysuitable fastening means. Examples of these means are snap-fastening,clamping by screwing, welding, etc. It is also advantageous for thepreassembled structure to bear means which make it possible to combinefurther accessories which, optionally, would be fixed later. These meansare also snap-fastening devices, tapped holes or threaded protuberancesof circular shape allowing screwing, surface regions suitable forwelding, etc.

The insertion of a preassembled structure may be combined with one ormore methods of fixing accessories already described above.

It is particularly advantageous for the preassembled structure alsoitself to bear at least one fuel tank accessory.

After the operation of joining the two shells together, the tank mayundergo any type of surface treatment. One example of treatment is thatwhich consists in fluorinating the tank. It is thus possible to treatthe internal surface of the tank or its external surface, or else totreat both surfaces simultaneously. After the molding operation, theshell advantageously undergoes deflashing of material, for example ofweld beads, in the regions which have been compression-molded. Thisoperation is generally performed with a profiled knife matched to theexternal dimensions of the tank.

According to the invention, it is beneficial, so as to improve theaccuracy of the dimensions of the shells, to then apply a shapingtemplate around the shells and to leave it in contact with at least oneportion of the regions of these shells for the time needed for thelatter to cool. This technique is particularly beneficial when thetemplate is in contact with the joining regions of the shells.

The figure below is given for the purpose of illustrating one specificmethod of implementing the invention, without wishing in any way torestrict its scope. It represents an extrusion-blow-molding installationwith continuous extrusion used to produce automobile gas tanks. Themultilayer extrudate (1) comprises 5 layers, namelyHDPE/adhesive/barrier/adhesive/HDPE. The adhesive is maleicanhydride-grafted polyethylene. The tubular extrudate (1) leaving thecircular die, which is mounted on the extrusion head (2), is cut along ageneratrix using a steel blade (3) placed at the exit of the circulardie.

After the extruate (1) has been cut, it is bent back to form a sheet (1)which is guided by means of a system of inclined surfaces (4). The mold(5) then comes into an open position beneath the extrusion head (2). Themold (5) is then closed around the sheet (1) causing it to becompression-molded in a region located between the punch (5 a) and thedie (5 b); in line with the joint between the shells (10). The sheet (1)is then pressed against the surface of the die (5 b) owing to it beingimpelled by the air on the die side being evacuated through ventinglines (7) and pressurized air being inlet on the punch side via ablowing line (6).

A delivery tube (8) mounted on an actuator (9) has also been pressedagainst the inside of the sheet (1), while it was still in the meltstate, being directly welded to this internal wall.

What is claimed is:
 1. A process for manufacturing plastic hollow bodiesfrom two shells formed by molding, which are joined together, at leastone shell being produced by compression-molding a portion of a plasticsheet between a mold and a punch and by the remaining portion of thesheet being blow-molded in the region not compression-molded,characterized in that it is applied to the manufacture of a fuel tankand in the sheet is obtained in the same manufacturing line as the shellwhich will be produced from this sheet, by the cutting and opening anextruded parison of closed cross section.
 2. The process is claimed inclaim 1, characterized in that that portion of the sheet which undergoescompression-molding comprises the regions for joining the shellstogether.
 3. The process as claimed in claim 1, characterized in thatthe two shells are compression-molded and blow-molded at the same time,by using stacked double molds (sandwich molds or stack molds).
 4. Theprocess as claimed in claim 1, characterized in that, before joining, atleast one accessory of the fuel tank is inserted into the shell andfixed onto it.
 5. The process as claimed in claim 4, characterized inthat, during the operation of molding the shell, at least one particulardevice allows the simultaneous molding onto the shell of at least onefuel tank accessory by flow of the sheet.
 6. The process as claimed inclaim 4, characterized in that at least one additional device forovermolding the shell allows at least one accessory of the fuel tank tobe molded and fixed directly by the addition of material on one side ofthe wall of the shell.
 7. The process as claimed in claim 1,characterized in that the addition of material is performed by theinjection-molding a plastic composition.
 8. The process as claimed inclaim 4, characterized in that at least one device placed on the punchside or on the die side of the mold makes it possible to fix, duringmolding, at least one fuel tank accessory by depositing and welding theaccessory to the sheet or by partially covering the accessory with aportion of the sheet.
 9. The process as claimed in claim 1,characterized in that a preassmbled structure is inserted duringmolding, allowing the tank to be stiffened by simultaneously pressing oneach of the two shells and/or fixing at least one accessory of the tank.10. The process as claimed in claim 2, characterized in that, aftershell has been molded, the weld beads are deflashed and then a shapingtemplate is applied, which is left in contact with a least one portionof the regions of the shells for the time need to cool the latter and toimprove the accuracy of the dimensions of the shells.